The Maryland and District of Columbia AFL-CIO has thrown its support behind Sen. Brian E. Frosh for the Democratic nomination attorney general, giving the Montgomery County lawmaker a potentially important boost in his primary contest.

The labor organization -- whose union affiliates represent an estimated 200,000 Marylanders -- made the endorsement over the weekend, choosing Frosh over Dels. Jon Cardin of Baltimore County, Aisha Braveboy of Prince George's County and C. William Frick of Montgomery.

Maryland-D.C. AFL-CIO President Fred Mason praised Frosh in a statement as "the only choice for attorney general."

A spokesman for Frick, Andrew Feldman, charged that Frosh was so much the choice of labor as the favorite of Senate President Thomas V. Mike Miller and the state Democratic Party establishment.

"This is really Mike Miller's endorsement," Feldman said of the Calvert County Democrat, who has served as head of the Senate for 26 years -- longer than anyone in Maryland history. Miller has thrown his support behind Frosh, who has served as one of his committee chairs since 2003.

Feldman said his candidate had sought the AFL-CIO endorsement. The spokesman said he believes the federation would have endorsed Frick had Miller not intervened.

"Some unions chose not to risk what would have happened to them politically if they made the right choice," Feldman said.

Frick's spokesman said the AFL-CIO made the endorsement over the objections of its Building Trades member unions -- a statement confirmed by Chuck Graham, business agent of International Brotherhood of Electrical Workers Local 26 in the Washington area.

Graham said the building trades abstained from the endorsement vote Saturday because the construction unions believed the AFL-CIO was acting prematurely in backing a candidate when the primary is still about eight months off.

While Graham criticized the endorsement process, saying the convention had been packed by supporters of AFL-CIO leaders, he did not back up Feldman's complaints about Miller.

The retirement of longtime Sen. Barbara Mikulski sets off a heated competition among politicians who may be vying for the Senate seat long held by the popular Democrat. While no one has officially announced a bid, the list of possible successors is long: